Why can some marine polychaetes not survive sudden drops in salinity?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

annelid *The structure of polychaetes.* (Left) Free-moving polychaetes. (A) *Neanthes,* (B) *Nereis.* (Right) Tube-dwelling (sedentary) polychaetes. (C) *Amphitrite,* (D) *Sabella.*

Many marine polychaetes cannot survive sudden drops in salinity because they are stenohaline organisms that lack the physiological mechanisms to regulate water intake when their environment becomes diluted.123

Typically, the body fluids of these marine worms are isosmotic with seawater, meaning they share the same salt balance as their surroundings. When the external salinity drops significantly, water moves into the worm's body through the process of osmosis because the worm cannot actively control or prevent this influx. Without specialized adaptations—such as the enlarged nephridia found in some tolerant species like Nereis diversicolor that allow for the excretion of excess water—the worm's body wall will swell and eventually burst.12